743 steadily employed men, age 40-65, have been examined comprehensively, with twenty-four hour ECG recordings, studied repeatedly for disorders of cardiac conduction and dysrhythmias, other evidences of heart disease, and potential risk factors and followed prospectively six to ten years with examinations at intervals of six months to four years. One hundred and twenty-one deaths have been investigated intensively, with observational data at the time of death in 114 cases. 74 arrhythmic deaths have been associated with chronic myocardial disease in 95% of cases (ischemia 84%, hypertrophy 83%, dilatation 22%, congestive heart failure 41%, heavy alcohol intake 29%), with acute myocardial disease in 77% of cases (acute ischemic heart disease 59%, sub-acute CHF 7%, acute anoxemia 28%), with chronic disorders of cardiac conduction system in 85% of cases (disorders of rate 34%, of pacemaker 30%, major supraventricular dysrhythmias 20%, conduction delays 32%, prolonged QT 23%, major ventricular dysrhytmias 48%; median, two of these), and with CNS arousal and physical activity. 253 variables in 12 categories of myocardial disease, disorders of rhythm and conduction, other cardiovascular disease, disorders of other organ systems, metabolic disorders, generalized pathological states, intake, premonitory symptoms, activity, behavior, social categories and age, and other general conditions are being analyzed in an attempt to define better the factors that are most predictive of the occurrance of sudden arrhythmic death, death in power failure, and new events of non-fatal heart disease.